"branch updater" not working?

After several days of tests around "branch updater" mode, I found that a change in master branch will not be merged into the dev branch in Bamboo automatically at all.

As introduced by Bamboo official documents:

"The Branch Updater should be used when you want to:

Automatically merge changes from the team's master branch into your feature branch, after a successful build of the master branch."

whenever I pushed a new commit to master branch in REPO, it does successfully trigger a build on master branch in BAMBOO build plan, but neither the change in master nor the successful build on master branch will trigger what "branch updater" promised to do, in expectation, the change in master or the successful build on master branch should "trigger a merge from master branch to dev branch in bamboo, then push it back to repo dev branch after a successful build."